Motors are commonly used to drive a variety of devices. Some motors move devices between two positions, such as motors that open and close the tray on compact disc (CD) and digital video disc (DVD) players. These motors may be operated to open or close the tray until a switch is triggered that shuts power to the motor (i.e., when the tray is fully opened or closed). Other motors, however, may be used to drive devices between a variety of different positions, and therefore a simple switching scheme may be inadequate to control the motor.
By way of illustration, one or more motors may be provided to drive the cartridge-access device of a media storage library to the various cartridge storage slots and to the read/write device therein. These motors may need to be more accurately controlled for efficient use of the media storage system. For example, where the user desires to write and/or read data on a particular data cartridge, the position of the cartridge-access device must be known so that it may be moved adjacent the desired cartridge storage slot to retrieve the desired data cartridge, and then moved to the read/write device to load it therein for read and/or write operations.
Encoders/decoders are available for determining rotational movement of a motor, and hence the position of a device being driven by the motor (e.g., the cartridge-access device). An encoder is typically mounted to a rotating member operatively associated with the motor (e.g., a motor shaft) and may have index markings thereon indicating fractional (or complete) revolutions of the rotating member that are read by the decoder to determine the rotational movement of the motor. However, the encoder must be assembled so that it is accurately spaced from the decoder, usually within very tight tolerances (e.g., within 0.015 inches of the desired spacing). Failure to assemble the encoder within the required tolerance may cause the decoder to improperly focus on the index markings, and may thus result in false readings and improper positioning of the driven device.
The encoder may be assembled on the rotating member using finely-calibrated machinery. For example, an assembly arm may be calibrated to press an encoder wheel onto the rotating member so that the encoder is assembled at the desired spacing from the decoder. However, this machinery is expensive and must be accurately calibrated before use, and frequently checked for proper calibration during use, which is time-consuming and slows production. Alternatively, the encoder may be manually assembled by measuring the distance between the encoder and decoder as the encoder is pressed onto the rotating member. However, this is a time-consuming process and is prone to human error. Often, the encoder cannot be manually assembled within the tight tolerances required by some encoder/decoder assemblies.